This invention relates to an operating mechanism for a manually operated bolted pressure contact switch of the type actuated by an over center spring drive in which the operating mechanism eliminates "hang up" of the switch contacts by preventing the spring drive from stopping in a fully compressed "dead center" position.
Fused bolted contact switches are frequently used in service entrance equipment and other relatively high current applications; typically multi-pole switches of this kind may require interruption of currents of the order of 400 to 80,000 amperes. It is critically important that the contacts of these switches be opened and closed rapidly to minimize arcing and thus avoid pitting and deterioration of the switch contacts. Because of the high currents handled by the switch contacts, they must not "hang up" in an intermediate position between fully open and fully closed or the contacts of the switch may be welded to each other by the flow of current.
The "hang up" of the switch contacts can occur when the over center spring drive hesitates or stops in a "dead center" position in which the drive spring is fully compressed during either opening or closing of the switch contacts. This unsafe condition can be brought about either unintentionally by a careless or inexperienced operator or deliberately by a thrill seeking or a malevolent operator.
Prior attempts to eliminate the dead centering of the spring drive have been concerned with modifications of the pivotal connection between the follower of the operating mechanism and the rod of the over center spring drive. An angled (non-symmetrical) elongated slot has been provided in the follower to receive a pivot pin connected to the spring drive rod so that as the spring drive reaches its "dead center" position (the fully compressed position of its spring) during opening of the switch contacts, the expanding spring force will shift the pivot pin in the angled slot past the "dead center" position of the spring drive. Unfortunately, the same result is not achieved with this prior construction during the closing of the spring contacts because the non-symmetrical angle of the elongated slot does not permit the expanding spring force to shift the pivot pin through the "dead center" position of the spring drive. This prior construction has not been entirely satisfactory for the additional reason that considerable wear can occur to the pivot pin as it moves in this angled slot, thus necessitating increased maintenance costs.